Lightweight asymmetric magnet arrays

ABSTRACT

A magnet array includes multiple magnet rings and a frame. The multiple magnet rings are positioned along a longitudinal axis and are coaxial with the longitudinal axis. At least one of the magnet rings encircles a predefined inner volume. A minimal inner radius of the magnet rings positioned on one side of a center of the inner volume along the longitudinal axis is different from the minimal radius of the magnet rings positioned on the other side of the center of inner volume. The magnet rings are configured to jointly generate a magnetic field of at least a given level of uniformity inside the inner volume. The frame is configured to fixedly hold the multiple magnet rings in place.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication 62/772,638, filed Nov. 29, 2018, whose disclosure isincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to magnet assemblies, andparticularly to lightweight magnet assemblies comprising permanentmagnets and design methods thereof.

BACKGROUND OF THE INVENTION

Designs of permanent magnet arrays aiming at achieving a strong anduniform magnetic field have been previously reported in the patentliterature. For example, U.S. Pat. No. 7,423,431 describes a permanentmagnet assembly for an imaging apparatus having a permanent magnet bodyhaving a first surface and a stepped second surface which is adapted toface an imaging volume of the imaging apparatus, wherein the steppedsecond surface contains at least four steps.

As another example, U.S. Pat. No. 6,411,187 describes adjustable hybridmagnetic apparatus for use in medical and other applications includes anelectromagnet flux generator for generating a first magnetic field in animaging volume, and permanent magnet assemblies for generating a secondmagnetic field superimposed on the first magnetic field for providing asubstantially homogenous magnetic field having improved magnitude withinthe imaging volume. The permanent magnet assemblies may include aplurality of annular or disc like concentric magnets spaced-apart alongtheir axis of symmetry. The hybrid magnetic apparatus may include a highmagnetic permeability yoke for increasing the intensity of the magneticfield in the imaging volume of the hybrid magnetic apparatus.

U.S. Pat. No. 10,018,694 describes a magnet assembly for a magneticresonance imaging (MRI) instrument, the magnet assembly comprising aplurality of magnet segments that are arranged in two or more rings suchthat the magnet segments are evenly spaced apart from adjacent magnetsegments in the same ring, and spaced apart from magnet segments inadjacent rings. According to an embodiment, a plurality of magnetsegments is arranged in two or more rings with the magnetizationdirections of at least some of the magnet segments being unaligned witha plane defined by their respective ring, to provide greater controlover the resulting magnetic field profile.

U.S. Pat. No. 5,900,793 describes assemblies consisting of a pluralityof annular concentric magnets spaced-apart along their axis of symmetry,and a method for constructing such assemblies using equiangular segmentsthat are permanently magnetized.

SUMMARY OF THE INVENTION

An embodiment of the present invention provides a magnet array includingmultiple magnet rings and a frame. The multiple magnet rings arepositioned along a longitudinal axis and are coaxial with thelongitudinal axis. At least one of the magnet rings encircles apredefined inner volume. A minimal inner radius of the magnet ringspositioned on one side of a center of the inner volume along thelongitudinal axis is different from the minimal radius of the magnetrings positioned on the other side of the center of inner volume. Themagnet rings are configured to jointly generate a magnetic field of atleast a given level of uniformity inside the inner volume. The frame isconfigured to fixedly hold the multiple magnet rings in place.

In some embodiments, the magnetic field generated by the array is alonga direction parallel to the longitudinal axis.

In some embodiments, the magnet rings are arranged with reflectionalasymmetry with respect to the longitudinal axis.

In an embodiment, each magnet ring has a rotational symmetry withrespect to an in-plane rotation of the magnet ring around thelongitudinal axis. In another embodiment, the inner volume is anellipsoid of revolution around the longitudinal axis.

In some embodiments, a given magnet ring is made of one of a singlesolid element and an assembly of discrete magnet segments.

In an embodiment, the magnet ring is pre-magnetized with a respectivemagnetization direction that maximizes uniformity of the magnetic fieldinside the inner volume. In another embodiment, the magnet ring ispre-magnetized with a respective magnetization direction that minimizesa fringe field outside the magnet array.

In some embodiments, the discrete magnet segments are electricallyinsulated from each other.

In some embodiments, each of the discrete magnet segments has a shapethat is one of a sphere, a cylinder, an ellipsoid and a polygonal prism.

In an embodiment, the discrete magnet segments are separated from eachother by at least one non-magnetic element comprising a solid, gas orliquid.

In another embodiment, at least two magnet rings contain a magnetizationvector in a direction different by more than 45 degrees from oneanother.

In some embodiments, each of the magnet rings has a shape including oneof an ellipse, a circle, and a polygon.

In some embodiments, the inner volume includes an imaging volume of anMRI system.

There is additionally provided, in accordance with an embodiment of thepresent invention, a method for producing a magnet array, the methodincluding positioning multiple magnet rings along a longitudinal axis,coaxially with the longitudinal axis, wherein at least one of the magnetrings encircles a predefined inner volume, and wherein a minimal innerradius of the magnet rings positioned on one side of a center of theinner volume along the longitudinal axis is different from the minimalradius of magnet rings positioned on the other side of the center ofinner volume, with the magnet rings configured to jointly generate amagnetic field of at least a given level of uniformity inside the innervolume. The multiple magnet rings are fixedly held in place using aframe.

The present invention will be more fully understood from the followingdetailed description of the embodiments thereof, taken together with thedrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an asymmetric magnet array comprising afirst magnet assembly and a second magnet assembly, according to anembodiment of the present invention;

FIGS. 2A and 2B-2D are a perspective view of an asymmetric magnet array,and plots of magnetic field lines generated separately and jointly bythe assemblies, respectively, according to another embodiment of thepresent invention; and

FIG. 3 is a perspective view of a segmented magnet ring, which may beany one of the rings in the magnet arrays of FIGS. 1 and 2, according toan embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS Overview

Magnetic fields that are strong and uniform are needed in a wide varietyof disciplines, spanning medicine, aerospace, electronics, andautomotive industries. As an example, magnets used in Magnetic ResonanceImaging (MRI) of the human brain typically provide a magnetic field witha strength of 0.1 to 3 Tesla, which is uniform to several parts permillion (ppm) inside an imaging volume of approximately 3000 cubiccentimeters, e.g. the interior of a sphere of radius 9 cm. However, suchmagnets have limited applications due to their considerable size andweight. Moreover, in general with magnet designs, there is a severelylimiting trade-off between weight, magnetic field uniformity, and a sizeof a volume inside which a given uniformity can be achieved.

Embodiments of the present invention that are described hereinafterprovide lightweight permanent magnet arrays that generate strong anduniform magnetic fields (e.g., in the range of 0.1 to 1 Tesla). Some ofthe disclosed magnet arrays are configured for emergency-care brainmobile MRI systems, such as a head MRI system inside an ambulance.Generally, however, the disclosed techniques can be applied in any othersuitable system.

In the description herein, using a cylindrical reference frameconsisting of longitudinal (Z), radial (r), and azimuthal (θ)coordinates, an inner volume is defined as a volume of an ellipsoid ofrevolution around the longitudinal axis. Examples of an inner volume area prolate having its long axis along the longitudinal axis, and anoblate having its short axis along the longitudinal axis. A lateralplane is further defined as any r-θ plane (i.e., a plane orthogonal tothe longitudinal z-axis). A particular definition of an inner volume isan imaging volume of an MRI system inside which the magnetic field hasat least a given level of uniformity.

In some embodiments of the present invention, a magnet array is providedthat comprises a frame, which is configured to hold, fixed in place,multiple magnet rings coaxial with a central longitudinal axis atdifferent positions along the axis, wherein the magnet rings lie inlateral planes with at least one ring encircling an area contained in aninner volume through which the longitudinal axis passes (i.e., the ringintersects the inner volume). In the present description a frame isdefined by its mechanical capability to hold the rings in place, andwhich can be made in various ways, for example, using a yoke or byembedding the rings in a surrounding material (e.g., in epoxy).

The multiple magnet rings are arranged with reflectional asymmetry withrespect to the longitudinal axis. In the context of the presentdisclosure and in the claims, the term “reflectional asymmetry withrespect to the longitudinal axis” means that no plane perpendicular tothe longitudinal axis is a plane of symmetry for the magnet array. Inother words, the magnet array is not symmetric under flipping withrespect to the longitudinal axis at any point along the axis.Reflectional asymmetry is also referred to as point asymmetry ormirror-image asymmetry. For brevity, any reference to “asymmetry” of themagnet array in the description below means the reflectional asymmetrydefined above.

The multiple magnet rings are configured to jointly generate a magneticfield along a direction parallel to the longitudinal axis of at least agiven level of uniformity inside the inner volume. The magnet array haseach magnet ring generate a magnetic field having a rotational symmetry(continuous or discrete) with respect to an in-plane rotation of thering around the longitudinal axis.

In some embodiments, each of the magnet rings of any of the disclosedmagnet arrays has a shape comprising one of an ellipse, most commonly acircle, or of a polygon. The magnet rings are each made of either asingle solid element or an assembly of discrete magnet segments. Themagnet rings are pre-magnetized with a magnetization direction which isdesigned to maximize the uniformity of the magnetic field inside theinner volume and optionally minimize the safety zone defined by the areaaround the magnet for which the magnetic field exceeds 5 gauss.

In some embodiments, which are typically configured for head MRIapplications, a disclosed asymmetric permanent magnet array can bedescribed as comprising a first magnet assembly, comprising two or moremagnet rings having a first inner diameter, and a second magnetassembly, comprising two or more magnet rings having a second innerdiameter. The first inner diameter is larger than the maximal lateraldiameter of the imaging volume and the second inner diameter is smallerthan or equal to the maximal lateral diameter of the imaging volume.

Typically, the magnet rings lie in different longitudinal axispositions. The second magnet assembly is asymmetrically placed relativeto the imaging volume. The asymmetric structure of the disclosed magnetarray is thus optimized to fit a human head, in which physical access toan inner volume (which is the same as the imaging volume) containing thebrain is through the first assembly but not the second. The first andsecond magnet assemblies are configured to jointly generate a magneticfield parallel to the longitudinal axis of at least a given level ofuniformity inside the inner volume.

The various types of magnet rings disclosed above are typically made ofa strongly ferromagnetic material, such as an alloy of neodymium, iron,and boron (NdFeB), whose Curie temperature is well above the maximumambient operating temperature. Other material options include ferrites,samarium-cobalt (SmCo) magnets, or any other permanent magnet material.Depending on the design and type of ring, ring segments may have theshape of a sphere, a cylinder, an ellipsoid, or a polygonal prism withshapes such as a cuboid, a wedge, or an angular segment.

The disclosed techniques to realize magnet arrays, separately orcombined, enable the use of strong and uniform magnet arrays inapplications that specifically require lightweight magnet solutions.

Asymmetric Magnet Array for Head MRI Applications

FIG. 1 is a perspective view of an exemplary asymmetric magnet array 100comprising a first magnet assembly 110 and a second magnet assembly 120,according to an embodiment of the present invention. As seen, first andsecond magnet assemblies 110 and 120 each comprise at least two magnetrings which are coaxial with a central longitudinal axis, denoted“Z-axis,” which passes through an inner volume 130. The multiplicity ofmagnet rings has variable transverse dimensions and variabledisplacements along the Z-axis. In FIG. 1, by way of example, firstassembly 110 is shown as consisting of four magnet rings, 111-114, andsecond assembly 120 is shown as consisting of four magnet rings,121-124. Each of the rings in assemblies 110 and 120 is either a solidring or a segmented ring, i.e., a ring comprising discrete segments. Thesegments may have the shape of a sphere, a cylinder, an ellipsoid, or apolygonal prism, preferably cuboids. It will be appreciated that therings may have any cross section including non-regular shape crosssection. All segments belonging to a single ring share a common shapeand material composition, as well as the same magnetic moment componentsin the longitudinal (Z), radial, and azimuthal directions. However, oneor more of these characteristics may differ from one ring to another.

In case of a segmented ring, referring to the magnetic moment of asegment means that the segment is uniformly magnetized to a specificdirection in space, its radial, longitudinal and azimuthal directionsare calculated in the segment center of mass.

In case of a solid ring, M varies continuously in space havingazimuthal, radial, and longitudinal components independent of theazimuth coordinate. It will be appreciated that a solid magnet piecewith a complex shape may be magnetized in a fashion that M_(r), M_(θ),or M_(Z) changes as a function of Z, or R, in a gradual or stepped way,creating effectively several rings from a magnetization perspective,although mechanically composed of one continuous piece. In the presentcontext, this sort of implementation is regarded as having multiplerings where their borders are determined by the magnetizationperspective, rather than by mechanical segmentation.

The peripheral shape of the rings may be any closed curve, such as acircle, ellipse, or polygon. In some cases, the choice of peripheralshape depends upon the cross-sectional shape of inner volume 130. Itwill be appreciated that a rotational symmetry of a ring, implies amongothers, that its peripheral shape is also rotationally symmetric (Forexample a shape of a circle, or an equiangular-equilateral polygon). Inthe special case where all rings are circular, the minimal inner radiusof rings 111-114 of first assembly 110 is denoted by R1, and the minimalinner radius of rings 121-124 of second assembly 120 is denoted by R2.For a given target radius Ri, which, by way of example, has the lateralradius 140 of inner volume 130 that defines a maximal radius of aspheroid volume inside that is used for imaging and which the magneticfield has at least a given level of uniformity, the values of R1 and R2satisfy the relationship Ri<R1, and 0≤R2≤Ri. In the case of R2=0, atleast one of the rings of second assembly 120 is a solid disc. It isappreciated that assembly 120 may contain rings with inner radius largerthan R2 and even larger than R1. The assemblies are separated in the Zdirection with a gap which is typically (but not limited to) 0-10 cm.For the present purpose, if a ring extends in Z direction to bothassemblies, one part of the ring will be considered as included in thefirst assembly while the other part in the second assembly. In this casethe gap between arrays will be 0.

In an embodiment, in the asymmetric array, the minimal radius of therings positioned on one side of the center of the inner volume isdifferent from the minimal radius of the rings positioned on the otherside of the center. The center of the inner volume can be defined in anysuitable way, e.g., the center of the section of the longitudinal axisthat lies within the inner volume. In addition, when the inner imagingvolume is only partially enclosed by the array the center will beconsidered as the center of the section of the longitudinal axis thatlies within the inner volume and inside the array. An array which obeysthe former embodiment may be described as comprised of twosub-assemblies with different minimal inner radiuses as described above.

Inner volume 130 is a simply-connected region at least partiallyenclosed by assembly 110, which is typically an ellipsoid or a sphere.As shown, the inner volume 130 is enclosed by the magnet array 110, withrings 112-113 encircling inner volume 130. In an embodiment, innervolume 130 is an oblate ellipsoid with semi-axes approximately equal to0.5 R1, 0.5 R1, and 0.3 R1. The parameters of such rings are not limitedto the inner and outer radius of a ring, its Z displacement, or Z-axisthickness. In addition, magnetic moment angles are all optimized using acalculation method such as a finite element, finite difference, oranalytical approach, combined with a gradient descent optimizationalgorithm to achieve the best uniformity, for a given field strength inthe imaging volume, with a minimal weight. This is allowed due to thefact that each assembly contains a multiplicity of rings, all of whichare optimized.

One aspect of the asymmetry of magnet array 100 is that different ringshave different transverse dimensions and magnetic moment directionswherein the rings are arranged in an array having reflectional asymmetrywith respect to the longitudinal axis (i.e., are asymmetrical withrespect to Z-axis inversion). In the context of the present disclosureand in the claims, the term “reflectional asymmetry with respect to thelongitudinal axis” means that no plane perpendicular to the longitudinalaxis is a plane of symmetry for the magnet array. In other words, themagnet array is not symmetric under flipping with respect to thelongitudinal axis at any point along the axis. Reflectional asymmetry isalso referred to as point asymmetry or mirror-image asymmetry. Forbrevity, any reference to “asymmetry” of the magnet array in thedescription below means the reflectional asymmetry defined above.

The asymmetry in the design is particularly advantageous when imaginginherently non-symmetrical specimens, such as the human head. Forexample, in one such case, it has been found that the rings belonging toassembly 110 may be primarily magnetized in a first given direction(e.g., the r-direction), whereas those belonging to assembly 120 mayprimarily magnetized in another direction (e.g., the z-direction).

Finally, the direction of magnetization of each individual ring may beoptimized to obtain both uniformity in the inner volume as well asfringe field reduction so as to create a magnetic circuit which closesthe field lines close to the magnet ring. In an embodiment, the discretemagnet segments are each pre-magnetized with a respective magnetizationdirection that minimizes a fringe field outside the magnet array.

FIGS. 2A and 2B-2D are a perspective view of an asymmetric magnet array200, and plots of magnetic field lines generated separately and jointlyby the assemblies, respectively, according to another embodiment of thepresent invention. Uniformity is not evident by uniform density of thelines (as lines were drawn denser in the imaging zone for betterdetails) rather by z-axis alignment of the lines.

As seen in FIG. 2A, an inner volume 230 is a simply-connected region atleast partially enclosed by a first magnet assembly 210, which istypically an ellipsoid or a sphere. A second magnet assembly 220 of theasymmetric array, “caps” inner volume 230. As mentioned above, differentrings may have different magnetization directions to optimize theuniformity and fringe field of the magnet array. For instance, one ringmay have a magnetization vector in a direction substantially different(e.g., by more than 45 degrees) from another ring. For instance, themagnetization vectors of the permanent magnet segments may pointprimarily in the r direction in one ring, and primarily in the Zdirection in another ring. Furthermore, two rings belonging to the sameassembly may have substantially different magnetization directions. Forinstance, one ring of the first assembly may have its magnetizationprimarily in the r direction, another ring of the first assembly mayhave its magnetization in primarily the −z direction while a third ringof the first assembly may have its magnetization at −45 degrees in ther-z plane. In an embodiment, the two or more rings have a magnetizationvector in a direction different by more than 45 degrees from oneanother.

In a particular case (not shown) it was found that the rings in assembly210 are dispersed in their inner radius between 15 cm and 30 cm, anddispersed in their Z position in a length of 25 cm, while the rings inassembly 220 are dispersed in their inner radius between 0.05 cm and 30cm, and dispersed in their Z position in a length of 12 cm, with thedisplacement between the two assemblies in the Z direction between 0 cmand 10 cm.

FIG. 2B shows the magnetic field lines of the field generated by firstmagnet assembly 210 (rings cross-sectionally illustrated by squares,each with a direction of magnetization of the ring in an r-z plane)inside and outside an inner volume 230. As seen, the field lines insideinner volume 230 are largely aligned along the z-axis, however theysharply bend at the top portion of volume 230, where the field becomesexceedingly non-uniform.

FIG. 2C shows the magnetic field lines of the field generated by secondmagnet assembly 220 inside and outside an inner volume 230. As also seenhere, the field lines inside inner volume 230 are largely aligned alongthe z-axis. However, they tilt opposite to the field lines of FIG. 2Bwith respect to the z-axis, and become exceedingly non-uniform at abottom portion of volume 230.

As seen on FIG. 2D, when combined into a full array 200, assemblies 210and 220 compensate for each other's field non-uniformity, to achieve auniform magnetic field along the z-axis to a better degree than aprespecified threshold.

FIGS. 2A-2D show an exemplary array containing ten rings. It will beappreciated that the array may contain more rings (e.g., several tens orhundreds of rings) which are all optimized as described above. The morerings contained in the array, the better magnet performance can beachieved (e.g., higher uniformity level, larger magnetic field or largerimaging volume). The improved performance comes with the drawback ofincreased complexity and production cost of the array due to the largenumber of elements. Thus, a practitioner skilled in the art shouldconsider the required number of rings according to the specificapplication.

FIG. 3 is a perspective view of a single segmented magnet ring 300,which may be any one of the rings in magnet arrays 100 and 200 of FIGS.1 and 2, according to an embodiment of the present invention. In FIG. 3,each magnet segment 310 has a magnetization vector 320 lying in the r-Zplane, with similar longitudinal (Z) and radial (r) components.Furthermore, each segmented ring possesses rotational symmetry with anazimuthal period equal to 360/N degrees where N is the number ofsegments in the ring. (For a solid ring, i.e., for N→∞, the rotationalsymmetry is continuous). In some embodiments, the disclosed rings haverotational symmetry of an order N≥8. It will be appreciated that thedisclosed array contains rings with rotational symmetry and hence theresult magnetic field is along the longitudinal axis. It is possiblehowever to incorporate in the asymmetric array rings which arenon-rotationally symmetric in a fashion that optimizes the fringe fieldand uniformity in the inner volume. In such a case the magnetic fieldmay be along an arbitrary axis. Although such an array may besubstantially worse than a rotationally symmetric array, the use ofasymmetry with rings as disclosed may substantially improve uniformityof the array compared to a symmetric one.

Discrete segments 310 are equally spaced and attached to one anotherusing, for example, an adhesive, which is preferably non-electricallyconducting, or are held together mechanically with gaps 330 betweenadjacent segments filled by (but not limited to) a preferably insulatingmaterial. It will be appreciated that the rotational symmetric segmentedrings may also include a combination of more than one type of segments.For thermal stability of all of ring 300, it is preferable that theadhesive or gaps consist of a material which is also thermallyconductive, such as silicon oxide, silicon nitride, or aluminum oxide.Individual magnet segments 310 may be made of the aforementionedstrongly ferromagnetic materials, whose Curie temperature is well abovethe operating temperature of an associated system that includes suchelements as an array 200, e.g., a mobile MRI system.

It will be appreciated that the descriptions in FIGS. 1-3 are intendedonly to serve as examples, and that many other embodiments are possiblewithin the scope of the present invention. For example, rotation of themagnet moment vector in the r-z-θ plane can be achieved, in analternative embodiment, by rotating the individual magnet segments 310through a distinct angle of rotation, which may be different fordifferent rings. Further, magnet arrays 100 and 200 may be combined witheither a static or dynamic shimming system, to further improve fielduniformity inside inner volumes 130 and 230, respectively. When dynamicshimming or gradient pulse fields are used, the presence of electricallyinsulating adhesive or empty gaps between adjacent magnet segments 310helps to minimize the negative effects of eddy currents on fielduniformity. Furthermore, magnet arrays 100 and 200 may be combined withresistive coils placed concentric to the z-axis, in order to enhance themagnetic field strength inside inner volumes 130 and 230.

The invention claimed is:
 1. A magnet array for use in a Magnetic Resonance Imaging (MRI) system, the magnet array comprising: multiple magnet rings, which are made of a permanent magnet material, and which are positioned along a longitudinal axis and are coaxial with the longitudinal axis, wherein at least one of the magnet rings encircles a predefined imaging volume of the MRI system, wherein the magnet rings are divided into (i) a first assembly characterized by a first minimal inner radius that is smallest among inner radii of the magnet rings of the first assembly, and (ii) a second assembly positioned alongside the first assembly along the longitudinal axis and characterized by a second minimal inner radius that is smallest among the inner radii of the magnet rings of the second assembly, wherein the first minimal inner radius of the first assembly is larger than the second minimal inner radius of the second assembly, wherein a center of the imaging volume is located outside the second assembly, and wherein at least one of the magnet rings in the first assembly has a non-zero inner radius and has a longitudinal magnetization component along the longitudinal axis, the magnet rings configured to jointly generate a magnetic field of at least a given level of uniformity inside the imaging volume; and a frame, which is configured to fixedly hold the multiple magnet rings in place.
 2. The magnet array according to claim 1, wherein the magnetic field generated by the array is along a direction parallel to the longitudinal axis.
 3. The magnet array according to claim 1, wherein the magnet rings are arranged with reflectional asymmetry with respect to the longitudinal axis.
 4. The magnet array according to claim 1, wherein each magnet ring has a rotational symmetry with respect to an in-plane rotation of the magnet ring around the longitudinal axis.
 5. The magnet array according to claim 1, wherein the imaging volume is an ellipsoid of revolution around the longitudinal axis.
 6. The magnet array according to claim 1, wherein a given magnet ring is made of one of a single solid element and an assembly of discrete magnet segments.
 7. The magnet array according to claim 6, wherein the magnet ring is pre-magnetized with a respective magnetization direction that maximizes uniformity of the magnetic field inside the imaging volume.
 8. The magnet array according to claim 6, wherein the magnet ring is pre-magnetized with a respective magnetization direction that minimizes a fringe field outside the magnet array.
 9. The magnet array according to claim 6, wherein the discrete magnet segments are electrically insulated from each other.
 10. The magnet array according to claim 6, wherein each of the discrete magnet segments has a shape that is one of a sphere, a cylinder, an ellipsoid and a polygonal prism.
 11. The magnet array according to claim 6, wherein the discrete magnet segments are separated from each other by at least one non-magnetic element comprising a solid, gas or liquid.
 12. The magnet array according to claim 1, wherein at least two magnet rings contain a magnetization vector in a direction different by more than 45 degrees from one another.
 13. The magnet array according to claim 1, wherein each of the magnet rings has a shape comprising one of an ellipse, a circle, and a polygon.
 14. A method for producing a magnet array for use in a Magnetic Resonance Imaging (MRI) system, the method comprising: positioning multiple magnet rings, which are made of a permanent magnet material, along a longitudinal axis, coaxially with the longitudinal axis, wherein at least one of the magnet rings encircles a predefined imaging volume of the MRI system, wherein the magnet rings are divided into (i) a first assembly characterized by a first minimal inner radius that is smallest among inner radii of the magnet rings of the first assembly, and (ii) a second assembly positioned alongside the first assembly along the longitudinal axis and characterized by a second minimal inner radius that is smallest among the inner radii of the magnet rings of the second assembly, wherein the first minimal inner radius of the first assembly is larger than the second minimal inner radius of the second assembly, wherein a center of the imaging volume is located outside the second assembly, and wherein at least one of the magnet rings in the first assembly has a non-zero inner radius and has a longitudinal magnetization component along the longitudinal axis, the magnet rings configured to jointly generate a magnetic field of at least a given level of uniformity inside the imaging volume; and fixedly holding the multiple magnet rings in place using a frame.
 15. The method according to claim 14, wherein the magnetic field generated by the array is along a direction parallel to the longitudinal axis.
 16. The method according to claim 14, wherein the magnet rings are arranged with reflectional asymmetry with respect to the longitudinal axis.
 17. The method according to claim 14, wherein each magnet ring has a rotational symmetry with respect to an in-plane rotation of the magnet ring around the longitudinal axis.
 18. The method according to claim 14, wherein the imaging volume is an ellipsoid of revolution around the longitudinal axis.
 19. The method according to claim 14, wherein a given magnet ring is made of one of a single solid element and an assembly of discrete magnet segments.
 20. The method according to claim 19, wherein the magnet ring is pre-magnetized with a respective magnetization direction that maximizes uniformity of the magnetic field inside the imaging volume.
 21. The method according to claim 19, wherein the magnet ring is pre-magnetized with a respective magnetization direction that minimizes a fringe field outside the magnet array.
 22. The method according to claim 19, wherein the discrete magnet segments are electrically insulated from each other.
 23. The method according to claim 19, wherein each of the discrete magnet segments has a shape that is one of a sphere, a cylinder, an ellipsoid and a polygonal prism.
 24. The method according to claim 19, wherein the discrete magnet segments are separated from each other by at least one non-magnetic element comprising a solid, gas or liquid.
 25. The method according to claim 14, wherein at least two magnet rings contain a magnetization vector in a direction different by more than 45 degrees from one another.
 26. The method according to claim 14, wherein each of the magnet rings has a shape comprising one of an ellipse, a circle, and a polygon.
 27. A magnet array for use in a Magnetic Resonance Imaging (MRI) system, the magnet array comprising: multiple magnet rings, which are made of a permanent magnet material, and which are positioned along a longitudinal axis and are coaxial with the longitudinal axis, wherein at least one of the magnet rings encircles a predefined imaging volume of the MRI system, wherein the magnet rings are arranged with reflectional asymmetry with respect to the longitudinal axis, wherein each magnet ring has a rotational symmetry with respect to an in-plane rotation of the magnet ring around the longitudinal axis, and wherein at least one of the magnet rings has a non-zero inner radius and has a longitudinal magnetization component along the longitudinal axis, the magnet rings configured to jointly generate a magnetic field of at least a given level of uniformity inside the imaging volume; and a frame, which is configured to fixedly hold the multiple magnet rings in place.
 28. A method for producing a magnet array for use in a Magnetic Resonance Imaging (MRI) system, the method comprising: positioning multiple magnet rings, which are made of a permanent magnet material, along a longitudinal axis, coaxially with the longitudinal axis, wherein at least one of the magnet rings encircles a predefined imaging volume of the MRI system, wherein the magnet rings are arranged with reflectional asymmetry with respect to the longitudinal axis, and wherein each magnet ring has a rotational symmetry with respect to an in-plane rotation of the magnet ring around the longitudinal axis, and wherein at least one of the magnet rings has a non-zero inner radius and has a longitudinal magnetization component along the longitudinal axis, the magnet rings configured to jointly generate a magnetic field of at least a given level of uniformity inside the imaging volume; and fixedly holding the multiple magnet rings in place using a frame. 